Superconducting magnet coil systems are operated in a cryostat in order to keep the temperature below the transition temperature of the superconductor. Typically, the cryostat has a vacuum vessel with one or more cryogenic vessels each containing a coolant, for example liquid helium or liquid nitrogen. The superconducting magnet coil system is installed in the coldest cryogenic vessel. This results in the superconducting magnet coil system being cooled in a highly temperature-stable and uniform manner. Systems of this kind may be used to cool NMR spectrometers in a bath, for example. In these systems, the vessels have to be refilled with the coolants at regular intervals because the heat input to the cryogenic vessels ensures that the coolants evaporate continuously. Alternatively, the coolants can be condensed by a cryocooler, or cooling may be achieved by thermally attaching the superconducting magnet coil system and/or one or more radiation shields of the cryostat to a cooling stage of a cryocooler.
In order to install an NMR probe, the vacuum vessel of the cryostat is typically provided with a room temperature access port into the cold bore of the superconducting magnet coil system. Since operating the NMR probe at colder temperatures improves signal quality, NMR probes may include cooled components. Various designs of NMR cryogenic probes of this kind are known. Usually, NMR cryogenic probes are attached in the room temperature access port of the cryostat so as to be removable. In this case, the cooled components may be arranged in a separate insulation vessel and may be cooled by a cooling circuit. NMR cryogenic probes that are at least in part fixedly mounted to the insulation vacuum of the cryostat are, however, also known.
Various cryogenic systems for cooling a superconducting magnet coil system and for cooling components of an NMR probe are known. These systems differ in particular with respect to the mechanical integration of the magnet assembly and NMR probe in an instrument-based unit, and with respect to the common use of components of the cryogenic system for cooling the magnet coil system and the NMR probe.
Some U.S. patent publications (US 2012/0242335 A1, US 2007/0107445 A1, US 2005/0202976 A1, and US 2006/0130493 A1 disclose assemblies comprising an NMR cryogenic probe that is attached in the room temperature access port of the cryostat of the magnet assembly so as to be removable.
In US 2007/0107445 A1, US 2005/0202976 A1, and US 2006/0130493 A1, the NMR cryogenic probe and parts of the cryostat of the magnet assembly are cooled by a common cryocooler.
In US 2007/0107445 A1 and US 2005/0202976 A1, helium gas from the cryostat of a superconducting magnet assembly is condensed at the helium gas outlet of the cryostat by a cryocooler. The cryocooler also cools the cooling circuit of an NMR cryogenic probe through heat exchangers. In US 2006/0130493 A1, a cryocooler is attached to the cryostat of a superconducting magnet assembly in a neck tube. The neck tube communicates with the helium vessel of the cryostat. Helium gas from the neck tube is guided through a cooling circuit into an NMR cryogenic probe. Helium gas is condensed at the bottom (coldest) end of the neck tube and flows back into the helium vessel of the cryostat.
These three assemblies are disadvantageous in that there is a high cost for cooling because the coldest stage of the cryocooler must be operated below the boiling point of liquid helium (4.2 K). Additionally, cryocooler vibrations are transferred to the cryostat because the cryocooler is attached directly to the helium gas outlet of the cryostat. Vibrations can affect NMR measurements.
In US 2012/0242335 A1, the NMR cryogenic probe is cooled by a cooling circuit that is thermally connected to a refrigeration reservoir of the cryostat of the magnet assembly, for example, to a nitrogen vessel. This assembly increases the cryogenic liquid consumption of the cryostat.
An assembly according to US 2012/0319690 A1 comprises an NMR cryogenic probe that is installed in the vacuum vessel of the cryostat of the superconducting magnet assembly. The magnet assembly and NMR cryogenic probe in this assembly are no longer mechanically modular. In order to replace the NMR cryogenic probe (for example, when there is a fault or in order to carry out NMR measurements that place different requirements on the functional scope of the NMR cryogenic probe), the cryostat vacuum has to be broken. Changing the NMR probe therefore requires that the superconducting magnet coil system be discharged and the magnet assembly be warmed.
The assembly according to EP 1 655 616 A1 or US 2006/0096301 A1 discloses an NMR apparatus having the following features:                superconducting magnet assembly        cryostat comprising a vacuum vessel        superconducting magnet coil system comprising a cold bore in which a room temperature access port of the cryostat engages        NMR cryogenic probe in the room temperature access or in the cold bore        NMR cryogenic probe comprising probe components cooled to an operating temperature of <100 K        at least two-stage cryocooler having the coldest cooling stage at an operating temperature of <30 K        cryocooler arranged in a separate, evacuated and heat-insulated housing so as to be spatially separated from the cryostat        cooling circuit comprising thermally insulated cooling lines between the heat-insulated housing and the NMR cryogenic probe.        
A two-stage cryocooler is located in a heat-insulated housing and cools the cooling circuit of an NMR cryogenic probe through heat exchangers and provides excess cooling capacity on the first (warmer) cooling stage. The excess cooling capacity condenses nitrogen gas from the nitrogen vessel of a cryostat, or cools a radiation shield of a cryostat of a superconducting magnet assembly. This assembly is disadvantageous in that the cooling capacity of the cryocooler cannot be used to reduce the evaporation rate of lower-boiling cryogenic fluids, such as liquid helium, because the temperature of the first cooling stage of the cryocooler is too high (approximately 35 K).